Electrostatic head



April 23, 1968 P. BETTS ETAL 3,380,070

ELECTROSTATIC HEAD Filed Feb. 19, 1964 QT/7r SIGNAL I F E 10 e SOURCE 26 5 58 INVENTORS IG 1 PETER BETTS F EVERETT T. EISELEN ATTORNEY United States Patent 3,380,070 ELECTROSTATIC HEAD Peter Bette and Everett T. Eiselen, San Jose, Calif., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Feb. 19, 1964, Ser. No. 345,912 7 Claims. (Cl. 346-74) ABSTRACT OF THE DISCLOSURE An electrostatic head consisting of a plastic block in which wire electrodes are embedded, the electrodes being positioned in ten groups of four, the groups being energized selectively according to the information to be printed and the members of the groups being energized equally and simultaneously. The head is bolted to a moving belt having conductive strips for connection to the electrodes and fastened opposite the head is a U-shaped spring which presses it against the paper at the print station.

This invention relates to an improved transducer for use in the art of electrostatic recording and, more particularly, to an electrostatic head construction capable of spot matrix recording on an adjacent electrostatically-sensitive medium.

In accordance with well known electrostatic printing techniques, electrical high-voltage pulse signals may be received by an electrostatic head and transformed thereby into electrical fields. These fields are caused to produce statically-charged areas upon the surface of a chargeable medium such as dielectrically coated paper as, for instance, the former moves past the paper station at which a grounded metallic surface contacts the back of the paper. The paper may then be moved to toning, transfer and fixing stations to provide a visible image and copies.

The pulse signals may represent characters in several Ways, one of the most common of which is by spot matrix. In this representation, the character is divided into a matrix of, for example, horizontal rows and 7 vertical columns comprising 70 sections. The head is composed of 10 sections, one corresponding to each row of the character matrix, and as the head moves over the portion of the recording station devoted to the character, each section of the head is pulsed separately in accordance with whether or not the character configuration requires a recording in the corresponding column. In this way a latent image of the character configuration is laid down on the surface of the dielectric paper.

The head electrodes are usually of the same configuration for the sections and it has been observed that best resolution of recording is obtained when they are pointed, i.e., in the form of wire ends directed towards the paper. This is believed to be the result of concentration of the electrostatic field in a narrow, collimated beam from the cross-sectional area of the electrode even Where pulsing is at high-speed.

However, it has also been observed that, although wire electrodes of small diameter produce characters having sections of clear delineation because of appreciable separation, readability is very poor, siimlarly shaped characters are distinguished with difficulty and the appearance of the print-out is of poor graphic quality. It is an object of the present invention to provide an electrostatic recording head which obviates these disadvantages.

It is another object of this invention to provide a head configuration which, when used in an electrostatic printer, provides satisfactory latent image recording despite variations in relative speed between the head and the record 3,380,070 Patented Apr. 23, 1968 medium, and despite changes in pressure of contact between the head and the record medium.

It is another object of this invention to provide an electrostatic head which may be driven to record well by pulses of amplitude and polarity reasonably expected as an output of electrical circuitry without requiring, in the printer, extraordinary facilities such as ground plane switching and biasing or paper preconditioning.

It is a further object of this invention to provide an electrostatic head structure characterized by ease of fabrication and adaptability to large scale economical production, of small size and weight adaptable to recording by being attached to an endless belt moved past the record medium at high speed, thereby capable of latent image recording at speeds commensurate with the receipt of the electrical pulses from the driving circuitry.

These objects and their attendant features characterize the electrostatic head of the present invention, which comprises a plurality of wire electrodes embedded in a plastic block, all electrodes being devoted to recording a 10- row column of the character matrix. Four electrodes are connected in parallel and are pulsed simultaneously in each row and thus comprise one stylus; the head therefore contains 10 styli of 4 electrodes each. The electrodes are evenly spaced, the ratio of electrode diameter to the distance between their centers being rigidly prescribed as determined by extensive experimentation to develop opti mum print-out quality. The writing surface adjacent the record medium is highly ground and polished; otherwise, dimensionally, ordinary machine tool tolerance is maintained. The head is adapted for fixing to a moving belt provided with conductive strips, one connecting to each stylus and to which commutation of drive signals may be made elsewhere on the belt. To insure good contact between the writing surface and the record medium, the head carries a loading spring on the other side of the belt.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIGURE 1 shows the head of the present invention associated with portions of an electrostatic printer which energize it and convey it past the record medium;

FIGURE 2 is an enlarged view of the head showing its mounting arrangement; and

FIGURE 3 is a view of the writing surface of the head in correspondence with a recorded matrix corresponding to the letter A.

For convenience and ease of understanding, the above figures will be referred to together in the description which follows.

Head 10 (FIGURE 3) comprises a rectangular block of suitable molding material of high dielectric strength, preferably of clear stable plastic, in which is embedded a plurality, here forty, of soft iron wire electrodes 12 each about 0.003 inch in diameter on 0.0075 inch centers. Electrodes 12 are arranged in sets of 'four to comprise a total of ten styli, such as stylus 16, in a single alignment along the long dimension of head 10. Stylus 16 is representative and will be referred to specifically although it should be understood that the description applies to the other nine styli as well. Stylus 16 is formed by pressure contact of its four electrodes at junction 18 (FIGURE 2) which connects it to conductive commutation strip 20. Commutation strip 20 is deposited, by electroplating or otherwise, on insulating belt 22; the material of the former may be copper and that of belt 22 may be thin tough plastic, such as polyethylene glycol terephthalate. For assurance of electrical contact and stability, head 10 is backed up on the other side of belt 22 by loading plate 28, to which it is attached by a pair of machine screws 30. Loading plate 28 is of thin spring steel having a warp as shown at edge 42 and is provided, at its edge opposite stylus 16, with cut-out loading fingers 32, one corresponding to each combination of stylus and its commutation strip, and, at its opposite edge, a curved extension, loading spring 34-. It is obvious that the configuration of loading plate 28 contributes a lock-washer effect for screws 30 as well as optimum contact between stylus 16 and its commutation strip in the vicinity of junction 18, and optimum closeness between head 10 and record medium 36 (FIGURE 1) backed up by ground plate 38 due to pressure against backing plate 40. As indicated, rotation of drums 41 and 43 sweeps head 10 past record medium 36 and commutation of signals from source 26 to head 10 is provided through commutation tabs, such as tab 39, mounted at drum 41 so as to contact the commutation strips, such as strip 20.

The recording matrix laid down by the head configuration selected for description herein is one of seven columns and ten rows shown in FIGURE 3, the electrostatic charge pattern for the letter A being illustrative. It is obvious that each stylus records a row of the matrix and, it may be surmised that single-electrode styli of large diameter would also provide satisfactory recording. However, considerable experimentation has demonstrated that singleelectrode styli produce printing inconsistent in quality; the styli at the center of the head do not contribute an electrostatic field as strongly as those at the ends. This etfect has been attributed to interaction between styli or charged areas and is minimized for each wire diameter by a minimum spacing of between 1 /2 to 3 times the diameter. This spacing is too distant for good legibility. However, it has been determined that it could be retained if the single-electrode stylus were split, preferably into a four-electrode stylus as in the head structure described above.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An electrostatic head capable of recording in the form of a spot matrix as it is conveyed past an electrostatically-sensitive paper positioned at a recording sta tion between a pair of backing plates, comprising:

a block of stable electrostatically inert material;

a carrier;

means to attach said block to said carrier; and

a plurality of styli embedded in said block and aligned along one dimension thereof, each stylus comprising a plurality of electrodes electrically connected together and to the carrier at one surface of said block and spatially separated at another surface of said block closest to the electrostatically-sensitive paper.

2. The electrostatic head of claim 1; and

a spring member associated with said attaching means,

said member including a set of spring fingers one opposite each of said styli and a spring extension extending between said block and one of the backing plates whereby said styli are pressed against the electrostatically-sensitive paper when traversing the recording station.

3. The electrostatic head of claim 1, wherein the electrodes of said styli are of the same diameter.

4. The electrostatic head of claim 3, wherein the spatial separation between the electrodes of said styli is approximately one and one-half times their diameter.

5. The electrostatic head of claim 4, wherein said styli comprise four electrodes in rectangular array.

6. An electrostatic head assembly of the type capable of recording with respect to an electrostatically sensitive surface in a matrix pattern of one or more columns and one or more rows, comprising:

a retaining block of stable, electrostatically inert material;

a plurality of Wire electrodes embedded in said retaining block and aligned in correspondence with a column of the matrix pattern;

means connecting at least some of the electrodes corresponding to at least one row of the matrix pattern for simultaneous and equal energization,

a carrier having continuous motion past the sensitive surface; and

means to mount the block on said carrier comprising at last one retaining device passing through said carrier into the block and a spring member including a set of spring fingers along an edge thereof, one finger corresponding to each connecting means, said spring member having a warped configuration for providing a lockwasher effect for said retaining device and concentration of pressure at its fingers.

7. The electrostatic head assembly of claim 6; and

means on said carrier connected to each electrode for individually energizing the electrodes with signals.

References Cited UNITED STATES PATENTS 1/1965 Waterman 346-139 7/1965 Benn 346-74 

